Metal halide lamps typically include a quartz arc tube having metal electrodes and a lamp fill material including halides of sodium, scandium or one or more of the rare earth metals, or combinations thereof. In addition, thorium oxide ThO2 and scandium Sc or cadmium Cd metals may be added to improve lumen maintenance.
Lumen depreciation and voltage rise in metal halide lamps are due in part to arc tube blackening, sodium loss or a loss of chemical species from halide reaction with the arc tube wall or electrodes.
Early metal halide lamps used pure tungsten electrodes which suffered from sputtering of the tungsten from the electrodes onto the arc tube wall during start-up, a high evaporation rate and the lack of a regenerative cycle during normal operation. Electrode material may also be chemically transported to the arc tube wall as halides.
Wall blackening has long been addressed by the doping of the electrodes with a suitable electron emissive material. The dopant reduces the work function of the electrode and results in a shorter glow-to-arc transition period and a lower electrode tip temperature. This in turn reduces the sputtering and evaporation of tungsten which causes blackening of the arc tube and lumen depreciation. Thorium oxide ThO2 in concentrations of 1% to 2% by weight is commonly used as the dopant, but is radioactive and difficult to manufacture.
The need for metal halide lamps with high efficacy, good lumen maintenance and long life is ever increasing. This has led to the development in recent years of sodium scandium metal halide lamps in which the arc tubes have a high wall loading to improve their performance. The increased arc tube loading has resulted in an increased voltage rise over the life of the lamp, a higher rate of lumen depreciation and a shorter lamp life.
In quartz metal halide lamps containing rare earth halides such as ScI3 and thoriated electrodes, a continuous increase in ThI4 content in the fill has been observed as the lamps are burned, thereby resulting in a continuous drop in light output over the life of the lamp. The present invention addresses the continuous increase of ThI4 in metal halide lamps with thoriated electrodes by eliminating the doping of the electrodes. The elimination of ThO2 in the electrodes reduces the chemical reaction of ScI3 in the fill with the ThO2 in the electrodes, and thus reduces the amount of ThI4 formed. The reduction of ThI4 reduces the operating voltage of the lamp.
Accordingly, it is an object of the present invention to obviate many of the deficiencies in the prior art and to provide a novel high pressure metal halide arc tube and lamp with good lumen maintenance and long life by eliminating the doping in the electrodes.
This and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.